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The effect of cerium concentration on single crystal lutetium orthosilicate (Lu2SiO5, LSO) was investigated with temperature-dependent steady state and time resolved luminescence spectroscopy. The results were used to determine the thermal quenching activation energies and the phonon energies responsible for the thermal broadening of the luminescence spectra. The measured phonon values were used to calculate configuration coordinate (CC) diagrams of the luminescence centers. The nature of single crystal LSO:Ce concentration quenching was determined to be due to radiative energy transfer by self-absorption. The observed broadening of the excitation spectra and narrowing of the emission spectra with increasing cerium was explained via a CC model. A combination of the CC model and concentration quenching explained the measured thermal quenching activation energies. [ABSTRACT FROM AUTHOR]

Cerium-doped YAlO3:Ce (YAP:Ce) is an interesting oxide scintillator that exhibits a wider range of light yield nonproportionality on a sample-to-sample basis than most other well-known oxide scintillators. In general, most oxide materials, such as Bi4Ge3O12 and Lu2SiO5:Ce, are thought to have an intrinsic proportional response that is nearly constant between samples and independent of growth conditions. Since light yield nonproportionality is responsible for degrading the achievable energy resolution of all known scintillators, it is important to understand what contributes to the behavior. In an attempt to understand if the phenomenon can be affected by growth parameters or by other means, seven samples of YAP:Ce were collected from various sources, and eight samples were grown in-house using the Czochralski method. Based on optical and scintillation measurement as well as direct measurement of the cerium concentration, it was determined that the light yield proportionality in YAP:Ce is strongly related to the cerium concentration. Samples that were found to have higher relative cerium concentration displayed a more proportional light yield response. In addition, it was determined that samples with higher cerium concentration also exhibit a faster decay time and an enhanced energy resolution when compared to samples with less cerium. It was also determined that growth in a reducing atmosphere can effectively suppress a parasitic optical absorption band. [ABSTRACT FROM PUBLISHER]

Time‐resolved spectroscopic study of the photoluminescence response to femtosecond pulse excitation and free carrier absorption at different wavelengths, thermally stimulated luminescence measurements and investigation of differential absorption are applied to amend the available data on excitation transfer in GAGG:Ce scintillators, and an electronic energy‐level diagram in this single crystal is suggested to explain the influence of codoping with divalent Mg on luminescence kinetics and light yield. The conclusions are generalized by comparison of the influence of aliovalent doping in garnets (GAGG:Ce) and oxyorthosilicates (LSO:Ce and YSO:Ce). In both cases, the codoping facilitates the energy transfer to radiative Ce3+ centers, while the light yield is increased in the LYSO:Ce system but reduced in GAGG:Ce. [ABSTRACT FROM AUTHOR]

GGAG:Ce crystals with various Ca concentrations were grown by the Czochralski technique. The introduction of Ca2+ ions into a trivalent site results in a change in the Ce valence state as well as an additional F+ luminescence center. The changes of Ce valence state could be affected by various annealing atmospheres and were investigated via measuring the Ce3+ absorbance and observing the color change of the sample. Photoluminescence spectra and photoluminescence decay were used to reveal the occurrence of F+ center related to the oxygen vacancies during the annealing. A redox mechanism and a charge compensation process are proposed to explain the change in Ce state charge and F+ center during the annealing. [ABSTRACT FROM AUTHOR]

In this work, the scintillation properties of Cs3LaCl6 and Cs3LaBr6 single crystals doped with various Ce concentrations (0.5 at.%–40 at.%) were studied. In the decay time profiles, both the fast (\sim 50 ns) and slow (\sim 500 ns) components decrease with increasing Ce concentration. The fast decay component dominates with high Ce concentration. Cs3LaCl6:Ce has a light yield of \sim 20\ 000 photons/MeV for 20 at.% Ce, while Cs3LaBr6:Ce has a light yield of 35 000 photons/MeV for 10% Ce. Energy resolution was improved from 20 at.% to 8 at.% with higher Ce concentration. Cs3LaCl6:Ce and Cs3LaBr6:Ce are 30 and 8 times less hygroscopic than LaBr3:Ce. [ABSTRACT FROM PUBLISHER]

In this paper, nine different samples of \ YAIO3:{\ {Ce}} have been collected and analyzed. The light yield non-proportionality of each sample was measured and used to classify each sample as proportional or non-proportional. A variety of scintillation and optical measurements were conducted on each sample, and the proportional samples were generally found to have a higher light output and better energy resolution. In addition, a strong linear correlation was found between scintillation decay time and the degree of non-proportionality. Based on absorption measurements as well as radioluminescence data, it was determined that the non-proportional samples all shared a range of increased absorption near the cerium 5d absorption edge between about 325 and 400 nm. The increased absorption has been reported in literature, and it is believed to be the result of a material defect introduced during growth. Thermoluminescence glow curves were measured for two representative \ YAIO3:{\ {Ce}} samples, one from each proportionality grouping, and it was determined that there was an observable change in defect structure, but there were no additional traps visible in the glow curves of either the proportional or non-proportional samples. However, the intensity of the 105 K thermoluminescence peak was found to be approximately a factor of two greater in the non-proportional samples. Since the lifetime of this peak is known to be between 25 and 81 ns, it was determined to be the likely cause of the slower decay in the non-proportional samples. [ABSTRACT FROM PUBLISHER]

The temperature dependence of the photoluminescence properties of Gd2SiO5:Ce,Ca crystals was found to be significantly affected by Ca codoping and shows that presence of additional energy levels in these crystals provides nonradiative energy transfer. The excitation and emission bands also broaden with codoping due to presence of these additional energy levels. Therefore Ca codoping in Gd2SiO5 crystals decreases the scintillation decay very strongly and has a strong adverse affect on scintillation light yield, different from the effect in other members of the oxyorthosilicate family such as Lu2SiO5 and Y2SiO5. [ABSTRACT FROM PUBLISHER]

Experimental studies of LSO:Ce crystals co-doped with various concentrations of Ca are presented. Photoluminescence decay time, excitation and emission spectra, thermal response and X-ray excited luminescence are investigated as a function of Ca concentration. Experimental data show Ca co-doping does not alter the energy level structure of either Ce1 or Ce2, but reduces the relative population of Ce2 to Ce1. Thus, the luminescence from Ce2 is suppressed, which contributes to the fast scintillation decay of Ca co-doped LSO:Ce. [ABSTRACT FROM AUTHOR]

Abstract: Lutetium oxyorthosilicate (LSO) thin films with a cerium thickness gradient were sputter deposited to investigate the optimum cerium concentration for emission intensity. Thin film cerium concentration ranged from 0.06 to 0.88at%. To compare the thin film samples to single crystal LSO, a set of single crystal LSO samples were investigated with cerium concentrations of 0.0015, 0.0095 and 0.078at%. The thin film samples showed peak photoluminescence emission intensity at a cerium concentration of 0.35at%; however, the single crystal samples exhibited peak photoluminescence emission intensity at a lower cerium concentration of 0.0095at%. The photoluminescence excitation and emission spectra as a function of concentration demonstrate the concentration quenching behavior and the mechanisms are speculated to be due to radiative (self-absorption) and non-radiative energy transfer, which may be phonon assisted. [Copyright &y& Elsevier]

Low temperature (∼35 K) measurements of the scintillation kinetics of Y2SiO5 : Ce (YSO:Ce) have previously illustrated that shallow electron traps can play an important role in the scintillation mechanism. In addition, divalent calcium co-doping of isostructural Lu2SiO5 : Ce (LSO:Ce) has been shown to eliminate shallow electron traps and decrease scintillation decay time while maintaining high light output. Here we investigate the effect of Ca2+ codoping on the trap populations and scintillation kinetics of YSO:Ce. Single crystals were grown with Ca2+ concentrations up to 0.5 at% relative to Y. Thermoluminescence measurements indicate a significant reduction in shallow traps, and a marked change in the scintillation kinetics can be seen in the scintillation time profiles as a result of Ca2+ codoping. [ABSTRACT FROM AUTHOR]

Cerium-doped lutetium oxyorthosilicate (LSO:Ce) is a well-known scintillator whose high density (7.4 g/cc), high light yield (∼6 times that of BGO), and fast decay time (∼43 ns) make it especially well suited for use in Positron Emission Tomography. Recent work suggests that divalent calcium co-doping eliminates many of the shallow electron traps, thus facilitating fast energy transfer to the Ce3+ luminescence centers and shortening the scintillation decay time to ∼30 ns. Here we report preliminary observations of the effect of Ca2+ co-doping on the crystal growth, photoluminescence, and scintillation properties of LSO:Pr. Single crystals with Pr concentrations ranging from 0.05% to 0.2% and Ca2+ concentrations up to 0.2% were grown via the Czochralski. technique. Absorption, emission, and excitation spectra revealed the numerous 5d-4f and 4f-4f transitions characteristic of trivalent Pr. The main optical absorption due to Ca2+ occurs in the far UV, similar to previous observations of LSO:Ce, but with a more in- tense tail extending through the visible region. Consequently, 4f to 5d optical excitation (∼260 nm) is greatly reduced in Ca co-doped samples, and the related Sd to 4f emission (∼400 nm) is largely absent. Optical excitation (∼450-500 nm) of the red (∼610 nm) emission appears to be unaffected, for the most part, by the presence of Ca2+. The scintillation decay of LSO:Pr is the sum of two exponential components with time constants of 7-8 and 26-29 ns, similar to previous reports. The addition of Ca2+ appears to relatively enhance the shorter component while reducing the longer one, although one or more additional slow components appear. [ABSTRACT FROM AUTHOR]

The renewed interest in Time-Of-Flight (TOF) Positron Emission Tomography (PET) has been accompanied by new research in the development of fast scintillators, mainly halides and/or lutetium-based compounds doped with Ce or Pr. In this work we measure some intrinsic properties of these materials, such as decay time and light output, which have a direct effect on time resolution, the key performance parameter for a TOF-grade detector. In particular, we report on measurements on LSO:Ce, LuAG:Pr, LuYAP:Ce, LaBr3:Ce and LaCl3:Ce. The scintillators are characterized in terms of absolute light yield, decay time, energy resolution, emission and excitation spectra, and time resolution. A new figure of merit to compare scintillators based on their performance in a TOF PET scanner is introduced. This figure of merit, the TOF effective sensitivity η, includes both interaction probability and timing characteristics. [ABSTRACT FROM AUTHOR]

The search for new scintillator crystals can be limited by the time consuming nature of the crystal growth process. In this paper, we demonstrate the use of a combinatorial thin film synthesis technique that is being used to explore new scintillator materials. The combinatorial synthesis process utilizes up to four individual R.F. magnetron sputtering sources which can be simultaneously powered to generate a wide composition space of binary, ternary, or quaternary material systems. In this work, we have investigated the lutetium oxide (Lu2O3)—silicon oxide (SiO2) material system doped with cerium. Lu2 SiO5 doped with cerium has been thoroughly characterized due to its use in PET imaging and provides a good benchmark for our proposed approach. Thin film, cerium doped lutetium oxide-silicon oxide gradients were synthesized to investigate the phases of the material system that exhibit scintillation properties and the results were compared to the results of bulk crystal samples. We have found that the emission spectra of the thin film materials have similar characteristics compared to the bulk crystals. Additionally, X-ray diffraction measurements have been correlated to the anticipated phases of the Lu2 O3 - SiO2 equilibrium phase diagram, and the intensity of the luminescence emission spectra have been correlated with the corresponding phases of the system. [ABSTRACT FROM AUTHOR]

Trivalent cerium ions form the luminescence centers in several important families of scintillation materials including the rare earth oxyorthosilicates, pyrosilicates, and aluminates. When comparing the experimentally determined scintillation properties of cerium-doped scintillators to theoretical models of scintillation mechanisms, there is often speculation regarding the fraction of the total cerium that exists in the radiative trivalent charge state (Ce3+) rather than the nonradiative tetravalent state (Ce4+). Until now, however, no technique has been developed to quantitatively measure both Ce3+ and Ce4+ in single crystal scintillators. We report here for the first time direct measurements of Ce3+ and Ce4+ in Lu2 SiO5 : Ce single crystals. Synchrotron radiation was used to measure the X-ray absorption on the M4 and M5 edges of Ce, and the results were compared to model samples of Ce3+ (Ce2O3) and Ce4+ (CeO2) which provided clear signatures of the two charge states. The spectra were obtained with a high-resolution superconducting tunnel junction spectrometer on beamline 4.0.2 at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The results clearly show 100% Ce3+, independent of light yield and sample coloration. Therefore, energy migration to the luminescence centers appears to be the determining factor in the scintillation efficiency of these samples, rather than variations in the Ce3+/Ce4+ ratio. [ABSTRACT FROM AUTHOR]

In recent years, the scintillation properties of several cerium-doped rare earth oxyorthosilicate scintillators, Ln2SiO5:Ce where Ln = Y, La-Lu, have been reported and, in some cases, extensively studied [11-[31. In addition, binary and ternary compounds such as (Lu,Y)2SiO5:Ce, (Lu,Gd)2SiO5 and (Lu,Y,Gd)2SiO5 :Ce have been reported [3]-[7]. All of these crystals have either monoclinic P or C structures with characteristic SiO4 tetrahedra and trivalent cations occupying two unique crystallographic positions [8], [9]. The trivalent cerium activator ions are assumed to occupy the cation lattice sites and possibly interstitial positions as well. The excited Sd state of Ce3+ is split into three observable levels with luminescence emission occurring only from the lowest 5d level to the 4f ground state (∼3 eV) with a Stokes shift of ∼0.5 eV. The bandgap is about 6 eV, and the index of refraction is close to 1.8, with some variation according to crystallographic direction. Despite these similarities, in somecases, differences in scintillation efficiency, decay time, rise time, and phosphorescence time profiles have been observed. In this paper, we combine thermoluminescence measurements with the temperature characteristic of scintillation efficiency between 10 and 350 K to determine the trapping levels that may influence the scintillation and phosphorescence properties of crystals of various compositions. [ABSTRACT FROM AUTHOR]

Commercially available Lu2SiO5:Ce (LSO:Ce) scintillators for the nuclear medical imaging applications, such as positron electron tomography (PET), normally have a light yield of 30 000–32 000 photons/MeV and a scintillation decay time of 43–45 ns. We demonstrate a simultaneous improvement of light yield and decay time of LSO:Ce single crystal scintillators with lithium codoping. Li codoping significantly enhances the light yield of LSO:Ce from 32 500 to 39 000 photons/MeV, shortens the scintillation decay time from 45.4 to 42.1 ns, and reduces the room temperature afterglow by around one order of magnitude. The physical insights on the role of Li codopants in scintillation mechanism are provided by studying cerium oxidation state, luminescence properties of cerium activator centers, defect structure, and preferential occupation of lithium ions. The improved light yield and afterglow are explained in terms of the dissociation of spatially correlated oxygen vacancies (VO) and Ce centers or the suppression of VO formation, allowing a more efficient electron migration to Ce centers. We attribute the decay time shortening upon Li+ codoping to the reduction of slow Ce2 emissions via a non‐radiative energy transfer from six‐coordinated Ce2 to seven‐coordinated Ce1 centers. This letter reports a simultaneous improvement of light yield, decay time, and afterglow of Lu2SiO5:Ce3+ scintillation detectors by Li+ codoping. Analysis reveals the non‐conversion of stable Ce3+ to Ce4+ upon Li codoping, and the critical role of Li site occupancy on the suppression of oxygen vacancies. This strategy is also expected to be extended to other oxide scintillators. [ABSTRACT FROM AUTHOR]

Active radiation environments and high-rate backgrounds pose a significant challenge towards improving the detection and characterization of Special Nuclear Material (SNM) using Associated Particle Imaging (API). Ongoing research seeks to reduce the sensitivity of a charged particle, or associated particle, detector to x-ray backgrounds through the creation and characterization of an alpha-sensitive, Ce-doped surface region of Yttrium Aluminium Perovskite (YAP). A wet-chemical layer of trivalent cerium (III) acetylacetonate hydrate was thermally diffused with a reducing atmosphere into single YAP crystals for a temperature range of 800 °C to 1500 °C. The diffusion concentration profile of the cerium compound was assayed with Rutherford Backscattering Spectrometry (RBS), where the Ce diffusion activation energy and the pre-exponential factor within YAP were found to be 1.02 ± 0.10 eV and 9.7 ± 2.0 × 10 - 15 m 2 s - 1 , respectively. Resulting transient polycrystalline structural features at the surface of the YAP crystals were investigated with X-Ray Diffraction (XRD). YAP, Yttrium Aluminium Garnet (YAG), and Yttrium Aluminium Monoclinic (YAM) phases were identified, as well as contributions from Y 2 O 3 , CeO 2 , Ce 2 O 3 , and the metastable hexagonal polymorph of YAlO 3 (YAH). The luminescent responses of the complex surface layers to x-ray and α excitement were examined as functions of annealing temperature. Radioluminescence emissions typical of the Ce3+ 4f → 5d transition occupying both the Y3+ and Al3+ sites in YAP and YAG were observed. The brightness of the samples under alpha radiation increased as a function of diffused Ce concentration until the point of optical darkening. [ABSTRACT FROM AUTHOR]

Single crystals of Ce-doped Cs 3 LaCl 6 and Cs 3 LaBr 6 show promising scintillation properties in gamma-ray detection. Their thermal response in the range from 40 K to 500 K is discussed in this work. The steady state photoluminescence spectra, photoluminescence decay time, and X-ray excited radioluminescence of Ce-doped and undoped Cs 3 LaCl 6 and Cs 3 LaBr 6 were studied. The photoluminescence and radioluminescence emission intensity of the Ce-doped crystals decreases less than 25% from 40 K to 500 K. The photoluminescence decay time of Ce 3+ increases with increasing temperature. [ABSTRACT FROM AUTHOR]

We recently discovered that mixed-anion Br–I elpasolite scintillators, Cs 2 NaYBr 3 I 3 : Ce and Cs 2 NaLaBr 3 I 3 : Ce, have promising performance. Ce concentration of these compounds was optimized in terms of light yield. Cs 2 NaLaBr 3 I 3 with 5% Ce (by mole) has a light yield of 58,000 ph/MeV, and excellent energy resolution of 2.9% at 662 keV. It is better than both endpoint compounds of the Br–I solid solution. Cs 2 NaYBr 3 I 3 with 2% Ce doping shows energy resolution of 3.3% at 662 keV, despite a relatively modest light yield of 43,000 ph/MeV. Non-proportionality of the mixed Br–I compounds was measured using gamma ray sources ranging in energy from 14 keV to 835 keV. The electronic band gaps of undoped Cs 2 NaLaBr 3 I 3 and Cs 2 NaYBr 3 I 3 were determined from optical transmittance and absorbance measurements. The band gaps of the compounds are 4.4 ± 0.1 eV, and 4.3 ± 0.1 eV, respectively. The crystal structures of Cs 2 NaLaBr 3 I 3 : Ce and Cs 2 NaYBr 3 I 3 : Ce are tetragonal and cubic respectively. The high symmetry leads to fewer cracks during crystal growth and minimizes light scattering at grain boundaries. The ease of crystal growth is promising for the scale-up of the growth process to larger sizes. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we report on optical and scintillation properties of a new scintillation material CsGd2Cl7:Ce. Crystals were grown in vacuum sealed ampoules, and the response to gamma rays and thermal neutrons was characterized. The scintillation light yield was ∼38,000ph/MeV, and the primary decay time was 60ns under γ-ray excitation. A thermal neutron response resulting from capture by 157Gd and 155Gd and subsequent emission of conversion electrons and X-rays was observed. The crystals exhibited a layered structure with cleavage planes and relatively low hygroscopicity. [Copyright &y& Elsevier]